In FIG. 1 is shown a cross-section of a conventional microstrip p-i-n travelling-wave microstrip electrooptic modulator. In such a modulator, an optical signal is transmitted within a region 10 formed in an electrooptic material such as not intentionally doped gallium-arsenic (GaAs) layer 11. In order to trap the optical beam within this electrooptic layer, this layer is sandwiched between a pair of layers (such as p-AlGaAs buffer 12 and n-AlGaAs buffer 13) having a smaller index of refraction than electrooptic layer 11. Thus, layer 11 functions as an optical waveguide. As a result of the electrooptic behavior of layer 11, the phase velocity of an optical beam in this waveguide is modified by an electric field applied in the waveguide. This velocity modification results in a phase shift in the optical signal exiting this waveguide. Therefore, the optical signal is phase modulated by the applied electric field.
The electric field in electrooptic layer 11 is produced by a voltage difference between a pair of electrodes 14 and 15. In a travelling wave modulator, the voltage difference between electrodes 14 and 15 is produced by a travelling wave electrical signal in electrode 14. This electrical signal is typically in the microwave range of frequencies.
In an ideal travelling wave modulator, the travelling wave electrical signal travels in electrode 14 with the same velocity as the optical signal travels in waveguide 11 so that any given part of the optical beam experiences a constant electric field as it travels along waveguide 11. Unfortunately, in general, the electrical signal in electrode 14 has a different phase velocity than the optical beam. As a result of the relative velocity between the electrical and optical signals, the phase modulation of any given point of the optical beam is proportional to the time integral of the electric field that it experienced during its transit through waveguide 11. This limits the bandwidth of the modulator. Therefore, to increase the bandwidth of such travelling wave modulators, it is necessary to increase the match between the electrical and optical signal velocities.
Numerous methods have been tried to deal with this natural mismatch of velocities. In one method (see, for example, M. Nazarathy, et al, "Velocity mis-match compensation in travelling wave modulators using pseudorandom switched electrode patterns", J. Opt. Soc. Amer., vol 4, pp. 1071-1079, 1987), there is no attempt to match velocities, but instead a mechanism is provided to compensate for the effects of the runoff of one of these waves relative to the other. In this method, the electrodes 14 and 15 are configured so that the polarity of the electric field through the waveguide alternates spatially along the waveguide in accordance with a pseudorandom code. These polarity reversals and some associated electronics enables a substantial compensation for the runoff of one of these signals relative to the other.
In another method (see, K. Kawano, et al, "High Speed and low driving power Ti:LiNbO.sub.3 Mach-Zehnder Optical Modulator at 1.5 micron wavelength, IEEE Lasers and Electro-optics Society 1988 Annual meeting, Nov. 1988 in Santa Clara, paper OEG.5), extra layers are added having an index of refaction selected to speed up or slow down one or the other of these signals. This method has been able to achieve substantial equality between these two velocities by moving the electrode away from the optical waveguide, but this results in a substantial increase in V.sub..pi. (the halfwave voltage). Such an increase in V.sub..pi. produces a substantial increase in power requirements which increase as the square of V.sub..pi.. If these two velocities could be substantially matched, then the bandwidth of the modulator would be substantially infinite. Thus, it would be very advantageous to have a travelling wave electrode design that would enable the phase velocity of the electrical travelling wave to be matched to the phase velocity of the optical signal without significantly increasing V.sub..pi..
The travelling wave modulator of FIG. 1 also exhibits an undesirably large amount of attenuation of the electrical signal. Because electrodes 14 and 15 are on opposite sides of a substrate 16, the distance d.sub.1 between these electrodes is too large for the characteristic impedance between these electrodes to equal the standard impedance of 50 ohms. Therefore, substrate 16 is sufficiently heavily doped that it is sufficiently conductive that the effective distance between electrode 14 and the ground plane is d.sub.2. Unfortunately, in order for the top surface of substrate 16 to function as the ground plane, a conduction current must travel from electrode 15 through substrate 16 to its top surface 17. Since this n+ region is still significantly resistive, it introduces an undesirable amount of attenuation of the electrical signal. Thus, an alternative design should also overcome this problem.